Search

The heat flux mitigation during the thermal quench (TQ) by the shattered pellet injection (SPI) is one of the major elements of disruption mitigation strategy for ITER. It's efficiency greatly depends on the SPI and the target plasma parameters, and is ultimately characterised by the heat deposition on to the plasma facing components. To investigate such heat deposition, JOREK simulations of neon-mixed dual-SPIs into ITER baseline H-mode and a 'degraded H-mode' with and without good injector synchronization are performed with focus on the first wall heat flux and its energy impact. It is found that low neon fraction SPIs into the baseline H-mode plasmas exhibit strong major radial plasmoid drift as the fragments arrive at the pedestal, accompanied by edge stochasticity. Significant density expulsion and outgoing heat flux occurs as a result, reducing the mitigation efficiency. Such drift motion could be mitigated by injecting higher neon fraction pellets, or by considering the pre-disruption confinement degradation, thus improving the radiation fraction. The radiation heat flux is found to peak in the vicinity of the fragment injection location in the early injection phase, while it relaxes later on due to parallel impurity transport. The overall radiation asymmetry could be significantly mitigated by good synchronization. Time integration of the local heat flux is carried out to provide its energy impact for wall heat damage assessment. For the baseline H-mode case with full pellet injection, melting of the stainless steel armour of the diagnostic port could occur near the injection port, which is acceptable, without any melting of the first wall tungsten tiles. For the degraded H-mode cases with quarter-pellet SPIs, which have 1 / 4 total volume of a full pellet, the maximum energy impact approaches the tolerable limit of the stainless steel with un-synchronized SPIs, and stays well below such limit for the perfectly synchronized ones. [ABSTRACT FROM AUTHOR]

Plasma disruptions pose an intolerable risk to large tokamaks, such as ITER. If a disruption can no longer be avoided, ITER’s last line of defense will be the Shattered Pellet Injection. An experimental test bench was created at ASDEX Upgrade to inform the design decisions for controlling the shattering of the pellets and develop the techniques for the generation of the fragment distributions necessary for optimal disruption mitigation. In an effort to analyze the videos resulting from the more than 1000 tests and determine the impact of different settings on the resulting shard cloud, an analysis pipeline, based on traditional computer vision (CV), was created. This pipeline enabled the analysis of 173 of the videos, but at the same time showed the limits of traditional CV when applied in applications with a highly heterogeneous dataset such as this. We created a machine learning-based (ML) alternative as a drop-in replacement to the original image processing code using a semantic segmentation model to exploit the innate adaptability and robustness of deep learning models. This model is capable of labeling the entire dataset quickly, accurately and reliably. This contribution details the implementation of the ML model and the current state and future plans of the project. [ABSTRACT FROM AUTHOR]

In this work we utilize the recently upgraded Thomson scattering diagnostic to resolve density and temperature plasma profiles after pure deuterium and mixed neon/deuterium shattered pellet injections (SPIs) on DIII-D. This allows us to study individual components of the staggered scheme proposed for disruption mitigation on ITER, consisting of a low-Z material SPI followed by a delayed high-Z SPI. Obtained spatio-temporal density profiles exhibit very different dynamics after dominantly neon and pure deuterium SPIs. The neon SPI causes a fast radiative plasma collapse in a few milliseconds and results in almost flat density profile once the impurity mixes with the plasma during and after the thermal quench (TQ). The deuterium SPI leads to a disruption delayed by ten and more milliseconds, but very limited core fueling can be observed before the disruption. Even during and after the TQ, the edge deuterium density significantly exceeds the core density. 1D transport modeling suggests that this poor core fueling can be explained by strong outward grad-B-induced drift of the injected deuterium. Preliminary simulations show that larger pellet shards and greater injected quantity can be used to improve the penetration of the low-Z material into the core. These results call for optimization and further evaluation of the staggered SPI on ITER. [ABSTRACT FROM AUTHOR]

A new control system of shattered pellet injection (SPI) has been successfully developed and implemented in the experimental advanced superconducting tokamak. The control system comprises four functional modules responsible for vacuum acquisition, temperature regulation, gas supply, and system protection, which facilitate the safe and stable operation of the SPI. The software framework employed for the SPI control system incorporates experimental physics and industrial control system and Phoebus. Utilizing these integrated control systems, the gun barrel temperature and material gas pressure could be accurately controlled during pellet forming phase. Also, it could cooperatively control the various types of valves to achieve material gas supply, propellant gas supply and timely pumping. Finally, the pellet was successfully generated, separated from the gun barrel, and accelerated into the plasma vacuum vessel controlled by this system. In addition, the issue of extended delay time was observed in SPI experiments, and a potential solution is also proposed in the paper. [ABSTRACT FROM AUTHOR]

Precise values for radiated energy in tokamak disruption experiments are needed to validate disruption mitigation techniques for burning plasma tokamaks like ITER and SPARC. Control room analysis of radiated power ( P _rad ) on JET assumes axisymmetry, since fitting 3D radiation structures with limited bolometry coverage is an under-determined problem. In mitigated disruptions, radiation is toroidally asymmetric and 3D, due to fast-growing 3D MHD modes and localized impurity sources. To address this problem, Emis3D adopts a physics motivated forward modeling (‘guess and check’) approach, comparing experimental bolometry data to synthetic data from user-defined radiation structures. Synthetic structures are observed with the Cherab modeling framework and a best fit chosen using a reduced χ ^2 statistic. 2D tomographic inversion models are tested, as well as helical flux tubes and 3D MHD simulated structures from JOREK. Two nominally identical pure neon shattered pellet injection (SPI) mitigated discharges in JET are analyzed. 2D tomographic inversions with added toroidal freedom are the best fits in the thermal quench (TQ) and current quench (CQ). In the pre-TQ, 2D reconstructions are statistically the best fits, but are likely over-optimized and do not capture the 3D radiation structure seen in fast camera images. The next-best pre-TQ fits are helical structures that extend towards the high-field side, consistent with an impurity flow under the magnetic nozzle effect also observed in JOREK simulations. Whole-disruption radiated fractions of $ 0.98 +0.03/ $ $ -0.29 $ and $ 1.01 +0.02/-0.17 $ are found, suggesting that the stored energy may have been fully mitigated by each SPI, although mitigation efficiencies well below ITER and SPARC requirements for high energy pulses are still within the large uncertainties. Emis3D is also used to validate JOREK SPI simulations, and confirms improvements in matching experiment from changes to impurity modeling. Time-dependent toroidal peaking factors are calculated and discussed.

Studies have been performed on the release mechanism for large pellets using high pressure gas in a shattered pellet injector. Typically, pellets are dislodged from the cryogenic surface and accelerated down a barrel using high pressure gas delivered by a fast-acting propellant valve. The pellets impact an angled surface which shatters the pellet into many small fragments before entering the plasma. This technique was initially demonstrated on DIII-D (Commaux et al 2016 Nucl. Fusion 56 046007) and is now deployed on JET, KSTAR, ASDEX-Upgrade, and other tokamaks around the world in support of ITER’s disruption mitigation system design and physics basis. The large hydrogen, 28.5 mm diameter, 2 length-to-diameter ratio, pellets foreseen for ITER SPI operation have low material strength and low heat of sublimation, which cause the pellets to be fragile and highly reactive to the impact of warm propellant gas. Due to the size of the pellets, significantly more propellant gas is required to dislodge and accelerate them. This creates a potentially significant propellant gas removal issue as 2–6 bar-L of gas is expected to be required for release and speed control. The research presented in this paper is an in-depth exploration of the parameters that are keys to reliable pellet release and speed control. Computational fluid dynamics (CFD) modeling of propellant flows through various breech designs was conducted to determine the force generated on the back surface of a pellet. These simulations assumed the use of the ORNL designed flyer plate valve. CFD modeling combined with experimental measurements provide adequate insight to determine a path to an optimal valve and breech design for ITER SPI pellet release and speed control while minimizing propellant gas usage.

The pre-thermal quench (pre-TQ) dynamics of a pure deuterium ( $\mathrm{D}_2$ ) shattered pellet injection (SPI) into a $3\,\mathrm{MA}$ / $7\,\mathrm{MJ}$ JET H-mode plasma is studied via 3D non-linear MHD modelling with the JOREK code. The interpretative modelling captures the overall evolution of the measured density and radiated power. The simulations also identify the importance of the drifts of ablation plasmoids towards the tokamak low field side (LFS) and the impurities in the background plasma in fragment penetration, assimilation, radiative cooling and MHD activity in $\mathrm{D}_2$ SPI experiments. It is found that plasmoid drifts lead to an about 70% reduction of the central line-integrated density (compared to a simulation without drifts) in the JET $\mathrm{D}_2$ SPI discharge considered. Impurities that pre-exist before the SPI as well as those from possible impurity influxes related to the SPI are shown to dominate the radiation in the considered discharge. With inputs from JOREK simulations, modelling with the Lagrangian particle-based pellet code PELOTON reproduces the deviation of the SPI fragments in the direction of the major radius as observed by the fast camera. This confirms the role of rocket effects and plasmoid drifts in the considered discharge and reinforces the validity of the JOREK modelling. The limited core density rise due to plasmoid drifts and the strong radiative cooling and MHD activity with impurities (depending on their species and concentration) could limit the effectiveness of LFS $\mathrm{D}_2$ SPI in runaway electron avoidance and are worth considering in the design of the ITER disruption mitigation system.

This study systematically explores the parameter space of disruption mitigation through shattered pellet injection in ITER with a focus on runaway electron (RE) dynamics, using the disruption modeling tool Dream . The physics fidelity is considerably increased compared to previous studies, by e.g. using realistic magnetic geometry, resistive wall configuration, thermal quench onset criteria, as well as including additional effects, such as ion transport and enhanced RE transport during the thermal quench. The work aims to provide a fairly comprehensive coverage of experimentally feasible scenarios, considering plasmas representative of both non-activated and high-performance DT operation, different thermal quench onset criteria and transport levels, a wide range of hydrogen and neon quantities injected in one or two stages, and pellets with various characteristic shard sizes. Using a staggered injection scheme, with a pure hydrogen injection preceding a mixed hydrogen-neon injection, we find injection parameters leading to acceptable RE currents in all investigated discharges without activated runaway sources. Dividing the injection into two stages is found to significantly enhance the assimilation and minimize RE generation due to the hot-tail mechanism. However, while a staggered injection outperforms a single stage injection also in cases with radioactive RE sources, no cases with acceptable RE currents are found for a DT-plasma with a $15\,\mathrm{MA}$ plasma current.

Shattered pellet injection (SPI) is a promising method for controlling plasma disruptions in tokamaks. In this study, we present numerical modelling of the fragmentation of cryogenic deuterium pellets within the context of SPI, using the peridynamic (PD) theory. A dedicated in-house code has been developed, leveraging the meshfree method and GPU parallelization. The mechanical properties of cryogenic solid deuterium are obtained from available literature, and calibrated based on the shatter threshold along with the remaining solid mass fraction after shatter. The results from the bond-based PD successfully reproduce the main experimental results reported in the literature, both qualitatively and quantitatively.

3D nonlinear MHD simulations of neon-doped single shattered pellet injection (SPI) conducted with the JOREK code reveal rich physics during SPI-induced disruptions in KSTAR. In the early phase, pressure-driven modes dominate, and the perturbation of the plasma current is largely consistent with the perturbation of the Pfirsch–Schlüter current. As shards reach the q = 1 surface, resistive current perturbations by helical electron cooling start to dominate, and the electron temperature in the core begins to collapse with convective mixing of the density driven by the internal $1/1$ kink mode. The confinement of the plasma is later completely destroyed as a cold bubble convects into the core via quasi-interchange mode. Comparisons with available experimental data demonstrate qualitative agreements between JOREK results and experiments, and possible reasons for deviations are discussed.

An effective disruption mitigation system in a tokamak reactor should limit the exposure of the wall to localized heat losses and to the impact of high current runaway electron beams, and avoid excessive forces on the structure. We evaluate with respect to these aspects a two-stage deuteriumâ€"neon shattered pellet injection in an ITER-like plasma, using simulations with the DREAM framework (Hoppe et al 2021 Comput. Phys. Commun. 268 108098). To minimize the obtained runaway currents an optimal range of injected deuterium quantities is found. This range is sensitive to the opacity of the plasma to Lyman radiation, which affects the ionization degree of deuterium, and thus avalanche runaway generation. The two-stage injection scheme, where dilution cooling is produced by deuterium before a radiative thermal quench caused by neon, reduces both the hot-tail seed and the localized transported heat load on the wall. However, during nuclear operation, additional runaway seed sources from the activated wall and tritium make it difficult to reach tolerably low runaway currents. [ABSTRACT FROM AUTHOR]

The analysis of the gas expansion in the test bench of the ITER DMS Support Laboratory is discussed. For this purpose, numerical FEM simulations were performed during the design phase of the laboratory. These are compared with experimental data, that have been obtained after the construction of the test bench was finished. The numerical and experimental results have been found to agree well. Both indicate the emergence of pressure waves in the acceleration barrel. These contribute to the pellet detachment from the barrel wall. Supersonic velocities have been also observed in the simulations, and an indirect indication for their presence has been found in the experimental results. The main processes during the propellant gas expansion inside the propellant gas recovery chamber are discussed, and a rough estimate for the amount of gas entering the flight tube of the test bench before the pellet is presented. • ITER Disruption Mitigation System Test Bench. • FEM simulation of propellant gas expansion. • Occurrence of pressure waves, shock waves. • Estimations for behaviour with moving pellet. • Experimental validation of simulations. [ABSTRACT FROM AUTHOR]

Dispersion interferometers (DI) are widely used to measure line integrated electron densities in many fusion devices. A recent development of a heterodyne single crystal DI (SCDI) with a laser wavelength of 1064 nm (Lee et al 2021 Rev. Sci. Instrum. 92 033536) allows an easier and simpler optical setup by using only one, instead of two, nonlinear crystal. It is found that the reported heterodyne SCDI with an acoustic-optical modulator (AOM) has different beam paths between the frequency-shifted, via the AOM, fundamental and second harmonics which act as the reference beams. Such a separation of the reference beams inevitably produces non-removable phase shifts associated with mechanical vibrations, resulting in a reduction of the removing efficiency of the mechanical vibrations that DI systems can provide. By utilizing the fact that the diffraction angle due to the AOM is inversely proportional to the frequency of the laser beam and linearly proportional to an order of the frequency-shift, the SCDI-Upgrade (SCDI-U), which has complete overlap of the optical paths for both probing and reference beams from the laser source to the detectors, is proposed in this work. Its first measurements in KSTAR during shattered pellet injections are reported, and results obtained by the SCDI-U are compared with those from the existing two-color interferometer (TCI) in KSTAR. It is found that the SCDI-U measures the electron density more reliably during such an abrupt and large density change than the TCI does. Qualitative analyses on the effects of different injection schemes of the shattered pellets and possible application of the SCDI-U for ITER are also discussed.

Recent studies suggest significant time delay between the Shattered Pellet Injection (SPI) fragment arrival and the temperature radiative collapse could exist in ITER, depending on the impurity assimilation and the plasma thermal reservoir. Hence in some cases the fragments could reach the core even before the edge radiative collapse occurs and triggers strong stochastic transport. This could be beneficial for heat load mitigation and hot-tail runaway electron suppression. To investigate the expected assimilation and radiation, thus the magneto-hydrodynamic (MHD) response after SPIs in 3D, we carry out simulations of collisional–radiative impurity mixed SPIs into ITER L-mode equilibrium. Localized cooling around the fragments is found to cause current perturbations which destabilize MHD modes. Meanwhile, slower injections are found to result in stronger and more complete radiative collapse, thus stronger MHD amplitude. Due to the q = 1 surface enclosing a significant volume, the $1/1$ resistive kink mode is shown to couple with outer modes to bring global stochasticity and convective core density mixing, although a transport barrier outside of the q = 1 surface prevents immediate temperature relaxation over the whole plasma. The impact of various physical assumptions and numerical treatments, such as the use of the flux-averaged ambient plasma parameters for ablation calculation, the exclusion of the magnetic constraining effect in ablation, the localization of the density source and the use of constant parallel thermal conduction instead of the Braginskii one and different injection velocities are also investigated. In general, stronger and more localized ablation results in stronger radiation, faster radiative collapse and a more violent MHD response, while the assimilation changes little due to a self-regulation effect.

Nonlinear 3D MHD simulations of shattered-pellet injection (SPI) in JET show prototypical SPI-driven disruptions using the M3D-C1 and NIMROD extended-MHD codes. Initially, radiation-driven thermal quenches are accelerated by MHD activity as the pellet crosses rational surfaces, leading to a radiation spike, global stochasticization of the magnetic field, and a complete thermal quench. Eventually, current quenches, preceded by a current spike are seen as the Ohmic heating becomes equal to the radiative cooling. The results are qualitatively similar for both a single monolithic pellet, pencil-beam model, and a realistic shatter to represent the SPI plume. A scan in viscosity from 500 to 2000 m ^2 s ^−1 for MHD simulations finds that reducing viscosity increases MHD activity and decreases thermal quench time slightly. A realistic cloud of fragments modeling shows that mixed-D–Ne pellet travels deeper into the plasma core before the thermal quench. At the slow pellet speeds, the pellet is found to be moving slowly enough inward that even the 5% neon in the mixed pellet is enough to effectively radiate the thermal energy available. Radiation toroidal peaking is predicted to be at levels consistent with experimental observations and reduced as the pellet travels deeper into the plasma. These simulations lay the ground work for more-sophisticated validative and predictive modeling of SPI in JET using both M3D-C1 and NIMROD.

One technique for mitigating disruptions in a tokamak is shattered pellet injection (SPI). SPI is a process in which a large solid pellet consisting of deuterium, neon, or argon is desublimated in a pipe gun barsrel and launched downstream. Pellets are shattered just before entering the plasma by an impact with an angled tube. Injection of these materials into the plasma radiates stored thermal energy, limits current decay rates, suppresses the generation of runaway electrons, and dissipates runaway electrons if necessary. A critical element of the SPI system is a fast-acting valve that releases high-pressure gas to dislodge and accelerate pellets directly, or indirectly via a mechanical punch. A prototype valve sized for the ITER SPI system has been designed and fabricated. A pulsed high-voltage power supply energizes the valve's internal magnetic coil, which induces eddy currents in the adjacent flyer plate resulting in a repulsive force between the flyer plate and the coil. The flyer plate action lifts a valve seat, allowing high-pressure gas to flow from the valve plenum to the downstream (breech) location of the pellet or mechanical punch. All of the valve's internal components are designed to operate in ITER-level static background magnetic fields. A study was conducted to optimize the downstream pressure response for a range of valve sizes and operating pressures. In particular, the study analyzes the breech pressure response associated with varying plenum pressures as well as varying breech volumes. A computational fluid dynamics simulation was built in STAR-CCM+ and validated against data from laboratory experiments. The resulting simulation outputs, in the form of downstream responses for a variety of initial plenum pressures and breech volumes, will be used as a complement to experimental data to ensure the pressure pulse is suitable for pellet survivability. These data, combined with studies on pellet shear strength and shock response, will be applied to optimization of overall operating parameters of the ITER SPI system. [ABSTRACT FROM AUTHOR]

Shattered pellet injection (SPI) has been chosen as the baseline disruption mitigation system on ITER due to its ability to rapidly inject material deep into the plasma to greatly increase the plasma density and radiate the thermal energy. SPI utilizes a mechanical punch or high-pressure gas to release and accelerate a pellet that has been cryogenically desublimated in the barrel of a pipe gun. Various material injection combinations could possibly be implemented during different phases of a disruption event to radiate plasma energy, reduce electromagnetic loads on machine components, avoid the formation of runaway electrons, or to dissipate runaway electrons that form. Each injection phase could possibly utilize combinations of deuterium, neon, or argon. In this paper we outline experimental measurements of pellet material shear strength at SPI operating temperatures to understand the force needed to release SPI pellets. Deuterium, neon, argon, and deuterium-neon mixture pellets with diameters of 8.5, 12.5, and 15.7 mm are formed at a range of relevant gas pressures and temperatures and dislodged from the cold zone with a slow-moving piston driven by a motor. The slow-moving piston is kept above the triple point temperature of the material while the pellet is forming, then cooled to below the triple point temperature before contacting the pellet to minimize any thermal conduction to the pellet. The piston incorporates a load cell to measure the force applied when the pellet breaks away from the cold zone in the barrel. The ability of the gas and punch methods to exceed the shear strength of the studied pellet materials for release has been analyzed. High-pressure gas delivered by fast-opening valves produce pressure shock to the pellet due to supersonic expansion of the propellant gas. Pressure (and therefore, force) oscillations are present due to transverse density propagation throughout the breech volume. Mechanical punches deliver an impact force through a high-kinetic energy impact. The effect of the mechanical shock on the pellet has been explored and is presented in this paper. Scaling to larger ITER-size SPI pellets will be described. [ABSTRACT FROM AUTHOR]

• Measurement of radiation during plasma disruption induced by shattered pellet injections in KSTAR • Heat transport on thin foil related to localized heating • Evaporation of carbon on metal foil in a high-energy environment • Film of multiwalled carbon nanotubes for high-temperature stability The thin-foil infrared sensor bolometer (IRSB) has been developed on the KSTAR to measure the rapid change of intense radiation during plasma disruption. Despites its successful commissioning, several issues have been identified regarding the interpretation of the measured time scale of the radiation and the durability of the foil. Concerning data interpretation, we propose an extended analysis model with the concept of effective heat capacity for a non-uniformly heated foil, allowing estimation of radiation power during plasma disruption. Concerning device durability, the carbon-coated foils are prone to surface evaporation in high-energy environment. As an alternative, we fabricate buckypaper foils composed of stable multi-walled carbon nanotubes, demonstrating high-temperature stability and superior light absorption. [ABSTRACT FROM AUTHOR]

Shattered pellet injector systems have been installed on DIII-D, JET, and KSTAR and used to experimentally determine the effectiveness of the shattered pellet injection (SPI) process in mitigating the deleterious effects of a tokamak plasma disruption. Pellets are fired, and before entering the plasma, strike a bent tube known as a shatter tube causing the pellet to shatter. The process of pellet fragmentation is a chaotic process that can be described in terms of fragment size distribution through a statistical model that incorporates the effects of the pellet material and impact characteristics. In addition to the fragment size distribution, the shatter plume has other characteristics of interest, such as a fragment velocity distribution and temporal mass evolution. The fragment velocity distribution is important because it is needed to accurately model the spread and location of the ablation and the deposition of impurities in the plasma over time. The temporal mass evolution is necessary to determine the time-resolved delivery of mass to the plasma. Due to installation constraints, the shatter tube currently installed on JET has a unique geometry with a modest S-bend followed by a 20-deg bend at the end of the tube. The DIII-D and KSTAR shatter tube design is a simple tube bent through an angle of 20 deg followed by a straight section. The resulting shatter sprays from the JET shatter tube and a 20-deg miter bend shatter tube were experimentally characterized for various pellet materials and speeds. Laboratory testing of these shatter tubes allows for the use of fast cameras to capture the fragment spray traveling through a large vacuum chamber. These high-speed videos of the shatter plumes allow the fragment size distribution, temporal mass evolution, and velocity distribution of the fragments within the plume to be determined. This paper presents a comparison of the unique geometry of the JET shatter tube to the miter bend geometries used for shattering and some insight into the variables that may be adjusted to produce the optimal shatter spray. The impact of entrained propellant gas on the resulting shatter spray was examined during testing. [ABSTRACT FROM AUTHOR]

Reliable mitigation is necessary to eliminate the detrimental effects of a disruption event in large high-current tokamaks such as ITER. To avoid serious damage to plasma-facing components during the thermal quench phase of a disruption, material is injected to radiate the plasma energy over the inner surface of the machine. The most promising method of material injection is a process known as shattered pellet injection (SPI). SPI utilizes cryogenic cooling to desublimate gas into the barrel of a pipe gun to form a solid pellet. High-pressure gas or a mechanical punch is used to dislodge the pellet and accelerate it into a bent tube to intentionally fracture it. Pellets made of a mixture of deuterium and neon are likely candidates for thermal mitigation. The survivability of these pellets throughout their flight path, before striking the shatter tube, is essential for reliable SPI operation. Experiments were conducted to determine intact speed limits for various mixtures. This paper outlines the details of brittle fracture theory and compares a theory-based model to experimental results from various mixtures of deuterium and neon pellets. [ABSTRACT FROM AUTHOR]

• Based on the experiences with our recently developed fast valve, a novel device has been designed to enhance durability, reliability, and to streamline the manufacturing and assembly processes. • Operation parameters of both the novel and the previous valve setup have been mapped and results have been compared. Results showed that due to the significant mass reduction, the novel setup can achieve the same pellet velocity while operating under less stringent parameters. This reduction in mechanical stress results in a more durable device. • The created operation maps serve as useful guides for setting valve parameters to achieve desired pellet velocities in SPI experiments. Furthermore, the methodology described in this contribution allows for the creation of operation maps for different valve versions and pellet types. Shattered Pellet Injection (SPI) is a common technique used in Disruption Mitigation Systems to prevent or minimize the effect of plasma disruptions in Tokamaks. To operate an SPI system, a fast-acting valve is needed for accelerating pellets. An eddy current actuated high-pressure fast valve was developed in 2021, dedicated to the SPI system of the ITER DMS Support Laboratory at the Centre for Energy Research. Based on the experiences of SPI operation, a novel fast valve has been developed to improve durability and reliability, while the design has been simplified in terms of manufacturing and assembly aspects. The results of the development and laboratory testing of this novel eddy current actuated fast valve are discussed in this contribution. [ABSTRACT FROM AUTHOR]

• Revealing trends in pellet acceleration by developing a zero-dimensional model. • The results are compared to the measurements of two different SPI test laboratory. • Low computational demand, thus fast design of injection scenarios. • Adiabatic propellant gas expansion without supersonic flow and wave propagation. Shattered Pellet Injection (SPI) is considered as a method to effectively mitigate the effect of severe disruptions in tokamaks. For the development of SPI technology in ITER, the Centre for Energy Research, in collaboration with H-ion Kft and VTMT Kft, designed, constructed and operates a Disruption Mitigation System (DMS) Support Laboratory. To simulate pellet acceleration, we developed a zero-dimensional model. The model uses the parameters of pellet material, propellant gas characteristics, dimension of the barrel and fast valve (FV), opening characteristics, temperature, and filling pressure of the FV in order to calculate the time dependent pellet position, pellet velocity, FV pressure and barrel pressure during the acceleration process. The goal of the acceleration model is to forecast velocities for parameters where no data is available as of yet. This paper describes the model and compares its results to two significantly different setups: the SPI measurements at the ASDEX Upgrade test laboratory and the ITER DMS Support Laboratory. We show that our model is capable of predicting tendencies and revealing the sticking stress of pellets to the barrel. [ABSTRACT FROM AUTHOR]

The ITER Disruption Mitigation System will consist of Shattered Pellet Injectors (SPI) which will be fired upon request into the ITER plasma to mitigate the detrimental consequences of disruptions. Each SPI will include a fast shutter to block the propellant gas after the pellet has passed to minimize the gas amount injected before the pellet, which was found to be critical to maintain disruption mitigation efficiency. An electromechanical test bench (ETB) was designed and built where the electromechanical model of the shutter actuator has been validated. Several material combinations were tested, to find the optimal solution for the fast shutter prototype. [ABSTRACT FROM AUTHOR]

• Preliminary design on multiple shattered pellet injection system for HL-3 has been performed. • In-situ condensation and propellant acceleration technologies are adopted for the dedicated multiple shattered pellet injection system. • Three pellets are prepared by three individual gas supply circuits at the same time, one of which is 7 mm and the others are 5.5 mm in diameter. • Steady-state and transient thermal analysis have been carried out to verifying the design scheme of pellet preparation system. In order to study the physical mechanism and control strategy of disruption mitigation, and to alleviate the destructive effects during disruption, a multiple shattered pellet injection system (multi-SPI) was designed for the HL-3 tokamak. This paper describes the preliminary engineering design of the multi-SPI, and the thermal analysis results of the pellet preparation system. In-situ condensation and propellant acceleration technologies are adopted for the multi-SPI. Three pellets can be prepared at one time and injected individually or simultaneously, one of which is 7 mm and the other two are 5.5 mm in diameter. As the key component of multi-SPI, thermal analysis of the pellet preparation system is given by ANSYS. Results show that the temperature distribution of the heat sink is relatively uniform, and the temperature of standby guns remains lower than the working gas triple point when the other guns were fired. The results verify that the structure design of the pellet preparation system can satisfy the design requirements. [ABSTRACT FROM AUTHOR]

TOKES simulations of shattered Ne pellets injection into the ITER discharge of 280 MJ thermal plasma energy was carried out taking into account shielding of the pellet. Injection was simulated to be performed through 3 upper port launchers or through 1 equatorial launcher. The shielding of pellets with 2 kPa m3 Ne has minor effects on the radiated power and on wall heat load because the injected amount is much larger than required for radiation. However, the minimum Ne amount, which radiates more than 90% of the core plasma energy (the threshold) is reduced by a factor of 2 compared to the result without shielding. The thresholds are 3 × 16 Pa m3 for upper injection and 62 Pa m3 for the equatorial injector. [ABSTRACT FROM AUTHOR]

Abstract Disruption mitigation by shattered Ne pellet injection (SPI) has been simulated for an H-mode ITER D-T discharge of 280 MJ stored energy using the 3D TOKES code. Heating of the first wall from the radiation flash during the thermal quench of the disruption initiated by the injected pellets has been assessed. The dynamics of Ne plasma expansion in the core with radiation from this plasma have been simulated for the series of pellet sizes from 5.4 × 1024 to 8.6 × 1022 Ne atoms. The threshold pellet size for irradiation of the core energy has been found. The results of SPI and massive gas injection simulations are compared. [ABSTRACT FROM AUTHOR]

• Dual shattered pellet injection (SPI) systems are installed in KSTAR to study disruption mitigation in support of the ITER DMS (Disruption Mitigation System) design activities. • KSTAR has two identical pellet injectors that can shoot pellets simultaneously from opposite toroidal locations. • The pellet integrity and velocity after formation have been measured using microwave cavity (MWC) systems and optical pellet diagnostic (OPD) system since 2021. • The commissioning for single and multiple injections of dual SPI is carried out. Dual shattered pellet injection (SPI) systems are installed in KSTAR (Korea Superconducting Tokamak Advanced Research) to study disruption mitigation in support of the ITER DMS (Disruption Mitigation System) design activities. KSTAR has two identical pellet injectors that can shoot pellets simultaneously from opposite toroidal locations. Each injector can shoot up to of three different diameter pellets. The pellet integrity and velocity after formation have been measured using microwave cavity (MWC) systems since 2019. However, this diagnostic provides only limited information on the pellet integrity, especially for partially broken pellets. Therefore, an optical pellet diagnostic (OPD) system monitoring the pellet has been installed in the first stage viewport of the differential pumping system and is operational since 2021. The KSTAR SPI systems offer a wide parameter range that must be characterized before the experimental campaign. To satisfy the experimental requirements, a proper procedure for pellet formation and acceleration must be commissioned. We carry out the commissioning by varying pellet parameters such as formation duration, temperature, heating, propellant gas pressure, and pulse length. The resulting pellet velocity, integrity, and tilt can be measured with the OPD system. These tests can be performed without plasma operation because no additional information is needed from the plasma response to the injection. This paper describes the results of the characteristic tests of single and multiple injections of dual SPI. [ABSTRACT FROM AUTHOR]

The ITER Disruption Mitigation System (DMS) Support Laboratory at the Centre for Energy Research, Budapest, Hungary aims at the characterization of the de-sublimation, acceleration and shattering of the cryogenic pellets, which will be fired upon request into the ITER plasma to mitigate the detrimental consequences of disruptions. The test bench includes a fast-acting valve for pellet acceleration, which releases high-pressure gas on a millisecond time scale. The valve is a new development since no valve on the market existed which would have met the requirements. The electromechanical modelling of the valve actuator has been validated against a laboratory experiment, the final system was built and thoroughly tested on a test bench to characterize the valve properties, and to demonstrate that the system fulfils the requirements. [ABSTRACT FROM AUTHOR]

Amongst the technological developments needed for ITER, the disruption mitigation system (DMS) is one of the most challenging since it has to mitigate the consequences of high-energy plasma disruptions to ensure the lifetime of ITER's structure and in-vessel components. ITER DMS is using Shattered Pellet Injection, whose technology needs to be developed for ITER specific requirements. In the framework of the "Technology Fundamental Studies for DMS" contract with ITER Organization, as part of the ITER DMS Task Force activities, and CEA, the DSBT (Low Temperature Systems Department) designed and operates a pellet injection test bench to produce and accelerate large cryogenic pellets with diameters from 10 to 28.5 mm and length/diameter ratios up to two. The objective of this activity is to study and to optimise the pellet formation and acceleration process using pure Protium, Deuterium, Neon and mixtures of these gases. Analytic and numerical studies were used to define the geometrical and cryogenic parameters for the pellet desublimation in the in-situ condensation cell of the cryostat. An analytical model has also been developed for the pellet acceleration in the single stage gas gun to validate the propellant valve design aiming at achieving velocities of up to 500 m/s for 28.5 mm protium pellets and 200 m/s for neon pellets. The complete test bench is composed of the gas propulsion system, the cryostat with its key component the pellet formation cell, the flight tube and the target chamber with viewing ports. The mechanical assembly is completed with specific instrumentation and fast data acquisition systems for the observation of the pellet formation and acceleration. The pellet integrity and trajectory are characterised by shadowgraphy with high resolution, high-speed cameras located at 1 m and 3 m after the barrel exit and with an accelerometer and a target foil in the target chamber. This document details the simulation studies, the selected test bench design and first obtained results. [ABSTRACT FROM AUTHOR]

• A barrel camera diagnostic for in-situ monitoring of 28 mm H pellets was developed. • The pellet formation can be followed real-time. • The material quality of the pellets can be determined using the barrel camera. • Low pressure and temperature during desublimation results foamy material. • Foamy pellets are easier to dislodge and are less brittle than solid ones. The ITER disruption mitigation system is based on shattered pellet injection. The ITER DMS requires the use of large, 19 × 38 and 28.5 × 57 mm (diameter x length) cryogenic pellets, made of hydrogen, neon, or a mixture of those. The production of such large pellets, especially protium, is not straightforward, therefore the freezing process is monitored in real-time. A barrel view camera diagnostic was developed at the Centre of Energy Research for this purpose: the pellet is illuminated from one side of the barrel via a special vacuum adaptor featuring a high-pressure rated window, while observing it from the other side using a digital camera. It was found that the growth of the pellet can be followed easily, and issues such as bad heat contact between the cold head and the barrel can clearly be observed as well. Once the pellet is launched, its integrity and other parameters can be determined in an optically assessable vacuum chamber, using a shadowgraphy diagnostic. The pellet size can routinely be determined to sub-mm precision, and voids (or other non-solid material regions) can be identified as dark areas on the captured shadowgraphs. [ABSTRACT FROM AUTHOR]

• The AXUV array diagnostics for shattered pellet injection experiments in KSTAR. • The poloidal AXUV arrays (PAAs) for poloidal cross-sectional radiation images. • The toroidal AXUV arrays (TAAs) for toroidal radiation distribution. • Strong radiation asymmetry for single SPI shots in KSTAR. • Radiated energy and CQ duration with various neon quantities in single SPI. Shattered pellet injection (SPI) is the most promising technique for disruption mitigation in reactor-scale magnetic fusion devices such as ITER. For SPI experiments in KSTAR, an AXUV array diagnostic system has been developed to obtain the radiation distribution on the poloidal plane and in the toroidal direction during disruption. AXUV photodiodes enable faster radiation measurement than the sub-millisecond scale. A tomographic reconstruction code for poloidal AXUV arrays has been also developed and tested by synthetic images. For single SPI experiments in 2020, AXUV measurement with a tomography technique visualizes radiation images on the poloidal plane during disruptions. For single SPI, the radiation is highly peaked at the SPI position. A neon (Ne) concentration scan for single SPI shots shows that the radiation energy and current quench duration change significantly with the Ne quantity and saturate at high Ne concentration. [ABSTRACT FROM AUTHOR]

Shattered pellet injection (SPI) is the technology chosen for the ITER Disruption Mitigation System and is explored at several fusion research devices, like DIII-D and JET and J-TEXT. The ITER disruption mitigation strategy relies on multiple injections to achieve RE (runaway electron) avoidance with optimum TQ (thermal quench), CQ (current quench) durations to adequately reduce wall loads. To demonstrate the feasibility of the multiple injection and to extrapolate to ITER, experiments with two identical injectors toroidally opposite to each other are needed urgently. KSTAR (Korea Superconducting Tokamak Advanced Research) can be a unique testbed to study the plasma disruption mitigation for ITER. KSTAR has installed two identical injectors in 180 degrees of toroidal opposite positions in 2019. For this system, ORNL (Oak Ridge National Laboratory) provided the two injectors, the shatter tubes, and auxiliary systems. NFRI (National Fusion Research Institute) provided the infrastructure of a vacuum pumping system, control & data acquisition system, and installed additional diagnostic systems for SPI in collaboration with the ITER Organization. This paper describes the engineering achievements during installation on KSTAR and the initial results of single and multiple SPI experiments in the 2019 campaign. [ABSTRACT FROM AUTHOR]

• TOKES numerical simulations of mitigated disruption thermal quenches, caused by SPI in ITER have been performed. • Bremsstrahlung and recombination radiation contributes significantly to the core cooling upon injection of mixed pellet • The minimum Ne amount in two mixed pellets, which radiates >90 % of the core plasma energy has been found. TOKES simulations of mixed Ne/D 2 pellet injection through 1 equatorial launcher into an ITER discharge of 280 MJ thermal plasma energy was carried out. Two kinds of pellets were simulated: 'large', containing 1.1∙1024 D 2 molecules and 'small' with 2.6∙1023 D 2 molecules, both with various amount of Ne and correspondingly lower quantities of D to preserve the pellet size. It was found that the physics of the core energy radiation by the mixed pellet is different from that of the pure Ne pellet. In case of the mixed pellet injection, bremsstrahlung and recombination radiation make a significant contribution to the core cooling along with the Ne line radiation, the latter being the only mechanism for pure Ne pellet injection. The minimum Ne amount in the mixed pellet, which radiates more than 90 % of the core plasma energy (the threshold) has been found to be 1∙1020 Ne atoms for the large pellet and 2∙1021 atoms for the small one. [ABSTRACT FROM AUTHOR]